(1) Field
The disclosed methods and systems relate generally to control techniques, and more particularly to control systems for materials manufacturing processes such as semiconductor manufacturing processes.
(2) Description of Relevant Art
Lithography is a process used in semiconductor manufacturing to transfer a circuit pattern from a photomask or reticle to a semiconductor wafer, or more specifically, to transfer the photomask pattern to a layer of resist that has been deposited on the wafer surface, where the resist is sensitive to irradiation. Different types of lithography can be based on the wavelength of the radiation used to expose the resist. For example, photolithography, otherwise known as optical lithography, uses ultraviolet (UV) radiation and a corresponding UV-sensitive resist. Ion beam lithography uses a resist sensitive to an ion beam, electron beam lithography uses a resist film sensitive to a scanning beam of electrons to deposit energy therein, and X-ray lithography uses a resist sensitive to X-rays.
Photolithography employs a photomask that can be understood to be a quartz plate that is transparent to UV radiation and includes a master copy of an integrated circuit that is often a microscopic integrated circuit. The photomask can be used to block resist exposure to select areas using chrome opaque areas.
A stepper is a resist exposure tool used in many photolithography systems to expose part of the wafer or resist in a given exposure. Systems employing a stepper can require a “step-and-repeat” process to expose the entire wafer as desired. A scanner is another type of resist exposure tool used in photolithography systems to expose part of the wafer or resist in a given exposure. Systems employing a scanner can require a “step-and-scan” process to expose the entire wafer as desired. In the aforementioned systems, overlay can be understood as the superposition of the pattern on the mask to a reference pattern previously created on the wafer surface. Related to overlay is alignment, which can be understood to be including positioning, or aligning, the mask or reticle relative to markers or targets on the wafer, prior to the exposure. Accordingly, to achieve proper exposure, overlay and alignment, among other parameters, must be properly controlled.
As the demand for smaller and more complex circuits increases, there is similarly increased demand for monitoring and hence improving overlay and alignment errors. Contributing to such errors can be the x-alignment of the wafer, the y-alignment of the wafer, the scale error or ratio of desired to actual stage movement in the x and y directions, the rotational error of the wafer, the reticle magnification error, and the reticle rotation error, among others.
To reduce these errors, it may be desirable to estimate reticle bias states related to overlay and critical dimension.